1. Field of the Invention
This invention relates to an image-forming apparatus such as an image display apparatus. It also relates to a method of manufacturing such an apparatus.
2. Related Background Art
CRTs (cathode ray tubes) are typical image-forming apparatus that utilize electron beams and have been used widely long since.
In recent years, flat type display apparatus have been gaining in popularity, replacing gradually CRTs. However, they are not emission type and accompanied by a number of problems including the need of a back light and hence there has been a strong demand for emission type display apparatus. While plasma displays are commercially available currently as emission type displays, they are based on a principle different from CRTs for light emission and are not comparable in terms of the contrast of the displayed image and the coloring performance of the apparatus. Meanwhile, efforts have been paid for research and development in the field of realizing a flat type image-forming apparatus by arranging a plurality of electron-emitting devices that is comparable with a CRT in terms of the quality of the displayed image. For example, Japanese Patent Application Laid-Open No. 4-163833 discloses a flat type electron beam image-forming apparatus realized by containing linear thermionic cathodes and complex electrode structures in a vacuum envelope.
With an image-forming apparatus comprising an electron source, the electron beams emitted from the electron source to strike an image-forming member can partly collide with the inner wall of the vacuum envelope to make it emit secondary electrons and become charged up to raise the electric potential at the local areas of the Inner wall hit by electron beams. Then, the vacuum envelope shows a distorted potential distribution to produce not only unstable electron beam trajectories but also internal electric discharges to degrade and eventually destroy the apparatus.
Known methods of preventing charge-ups include forming an anti-charge film on the inner wall of the vacuum envelope of the apparatus. Japanese Patent Application Laid-Open No. 4-163833 discloses an image-forming apparatus comprising an electroconductive layer of a high impedance electroconductive material arranged on the lateral sides of the inner wall of the glass envelope of the apparatus.
In an image-forming apparatus utilizing electron beams, a voltage is applied between the electron source and the image-forming member of the apparatus to accelerate electrons emitted from the electron source. If the vacuum envelope of the image-forming apparatus is made of soda lime glass or some other glass containing sodium (Na), Na ions are forced to move by the electric field that is generated by the applied voltage to give rise to an electrolyzing current. A vacuum envelope using glass is typically prepared by bonding a number of members by means of frit glass. As Na ions are forced to flow into the frit glass of the vacuum envelope by an electrolyzing current, PbO contained In the frit glass is reduced to deposit Pb and produce cracks in the frit glass so that the vacuum condition in the envelope can become damaged. A technique for preventing such a situation is to provide the vacuum envelope with an electrode at an appropriate location on the outer wall thereof to attract the electrolyzing current that can otherwise flow into the frit glass. For example, Japanese Patent Application Laid-Open No. 4-94038 proposes the use of a low resistance electroconductive film arranged along the periphery of the face plate and connected to the ground to prevent any electrolyzing current from flowing into the frit glass of the vacuum envelope. U.S. Pat. No. 5,357,165 discloses the use of a stripe-shaped electrode for causing an electric current to flow along the lateral wall of the vacuum envelope and producing a gradient of electric potential.
FIG. 15 of the accompanying drawings shows an equivalent circuit for the above known arrangement. In FIG. 15, spot 71 represents the image-forming member to which voltage Va is applied and spot 72 represents the junction of the components of the vacuum envelope, while resistor 75 has an electric resistance equal to that of the anti-charge film formed on the inner wall of the vacuum envelope between 71 and 72. Spot 73 represents the wire extending from the inside of the vacuum envelope to the outside through the junction of the components to drive the electron source and the electric resistance of the frit glass between 72 and 73 is equal to that of resistor 76. The wire is connected to the terminal 79 of the power source for driving the electron source that shows a given electric potential. The resistance of the wire is equal to that of resistor 80. The electrolyzing current flowing from the image-forming member 71 to the junction 72 through the inside of the glass of the vacuum envelope experiences electric resistance the magnitude of which is equal to that of the electric resistance of resistor 77. Reference numeral 74 denotes an electrode arranged outside the vacuum envelope for capturing the electrolyzing current. The electrolyzing current flowing through the inside of the glass encounters electric resistance having a magnitude equal to that of the electric resistance of resistor 78. The electrode 74 is connected to the ground by way of the resistance of the conductor connected to it. The junction 72 is connected to member 82 having a given electric potential, experiencing electric resistance whose magnitude is equal to that of the resistance of resistor 81.
Note that, while FIG. 15 shows a possible equivalent circuit for a known arrangement for avoiding charge-ups in an image-forming apparatus, it may not accurately correspond to the arrangement in a rigorous sense of the word.
However, a flat type electron beam image-forming apparatus as described in Japanese Patent Application Laid-Open No. 4-163833 is in fact not very flat and has a considerable depth because the glass envelope of the apparatus contains specifically designed structures including horizontal and vertical deflecting electrodes in it. On the other hand, there is a demand for electron beam image-forming apparatus to be used as portable information processing terminals that are as flat and light weight as a liquid crystal display.
In line with the efforts for realizing very flat image-forming apparatus, the applicant of the present patent application has achieved a number of improvements for surface conduction electron-emitting devices and image-forming apparatus comprising such devices. For example, Japanese Patent Application Laid-Open No. 7-235255 describes an electron-emitting device having a simple configuration. Such devices can be arranged over a relatively large area in large numbers to realize a very flat electron beam image-forming apparatus without using complex structures such as electrode structures
In an image-forming apparatus of the type under consideration, a voltage is applied between the electron source and the image-forming member to accelerate electrons. If ordinary fluorescers are used for the image-forming member, this voltage is desirably raised at least to a level of several kV in order to provide the emitted light with a desired coloring effect. Then, the use of a specifically designed voltage supply terminal having a connection structure that can prevent electric discharges and deal with high voltages will be required to apply a voltage of several kV to the image-forming member.
Meanwhile, a flat type electron beam image-forming apparatus requires a voltage supply terminal having a connection structure for applying a voltage to members within the vacuum envelope such as an anode that is structurally different from the connection structure of a CRT. As for connection terminals of the type under consideration, Japanese Patent Application Laid-open No. 5-114372 proposes an arrangement of using a metal rod running through the glass plate of the back side of the vacuum envelope, sealing the gap between the glass plate and the metal rod with frit glass and keeping the resilient front end of the metal rod physically in contact with the metal back layer of the image-forming section within the vacuum envelope. Japanese Patent Application Laid-Open No. 4-160741 proposes an arrangement of using a terminal connecting section connected to the inside of the vacuum envelope by means of an electroconductive adhesive agent. An arrangement of using a connection terminal connected to the inside of the vacuum envelope and drawn out through a lateral side of the vacuum envelope is described in Japanese Patent Application Laid-Open No 4-94038. Similar arrangements are disclosed in Japanese Patent Applications Laid-Open Nos. 4-98744 and 6-139965. Japanese Patent Application Laid-Open No. 4-94043 describes an arrangement for a connection terminal that runs through a through hole which is bored through the face plate and is connected to the inside of the vacuum envelope.
With any of the above arrangements, the terminal is connected to a high voltage feed-in wire within the vacuum envelope. Now, the vacuum envelope is exposed to high temperature during the operation of assembling it as frit glass applied thereto to hermetically seal the components of the vacuum envelope. Then, the junction of the high voltage feed-in wire and the connection terminal is also exposed to high temperature so that, If an adhesive agent is used to the junction, the impurities contained in the adhesive agent can be released therefrom to adversely affect the electron emitting performance of the apparatus. If, on the other hand, they are held resiliently in contact with each other, the resilience of the resilient member can be degraded and/or a defective connection can occur due to faulty handling or fitting operation during the assembling process. Once the image-forming apparatus is assembled, it is practically impossible to correct the connection if it is found faulty so that the time and labor consumed for the assemblage is reduced to nil to lower the manufacturing yield of the line.
Thus, the reliability of the connection of the high voltage terminal within the vacuum envelope is not highly reliable and a poor reliability inevitably reduces the manufacturing yield of the line. Particularly, if the connecting section where a high voltage is fed is faulty, the entire image-forming apparatus can remain inactive to make the apparatus useless. To avoid such a situation, a rigorous line control system will have to be implemented to raise the cost of controlling the line.
The arrangement of providing the flat type image-forming apparatus with a projection at a lateral side for electric connection is also accompanied by problems including that the cabinet holding the apparatus is forced to become bulky in order to accommodate the projection if the apparatus is a TV set. While this problem may be alleviated when the projection is located at the front or rear side, problems may also arise in terms of the design of the cabinet, the assembling process and so on to raise the manufacturing cost.
Another problem for a flat type image-forming apparatus to cope with a high voltage is a high risk of electric discharges that can occur along the inner wall of the vacuum envelope due to the reduced distance between the image-forming member and the electron source of the apparatus. A very large electric current is flows instantaneously when an electric discharge occurs and the electron-emitting devices of the electron source can be subjected to a very high voltage when the electric current flows, if partly, into some of The wires of the electron source. When the voltage exceeds an allowable level for the normal operation of the electron-emitting devices, their performance can become degraded and, in some cases, some of the devices can become destroyed. Then, the image displayed on the image-forming apparatus can be lost, if partly, to remarkably degrade the quality of the image and make the image-forming apparatus no longer operational.
Thus, the problems to be solved for arranging a connection terminal on a flat type electron beam image-forming apparatus include the following.
(1) It ensures a reliable electric connection. PA1 (2) It does not need a projection at a lateral side of the vacuum envelope. PA1 (3) It does not adversely affect the atmosphere in the vacuum envelope.
Thus, there is a strong demand for highly reliable flat type electron beam image-forming apparatus that are free from the above identified problems.